1. Field of the Invention
The present invention relates to an art for generating a clock signal. More particularly, it relates to a device, for example, a programmable timer, for generating a clock signal, of which duty ratio is nearly 50%, on the basis of a reference clock in a communication system or the like.
2. Description of the Related Art
Along with an increase in the operating speed of microprocessors, efforts have been made to realize a higher transmission speed and improve the precision in control of a device. Accordingly, there is a growing demand for signals, such as communication clocks that have high frequencies and high precision. Signal generators employed are therefore required to be flexible enough to enable setting of an output frequency.
As far as a known programmable timer is concerned, a reference clock is fed to a timer unit. An output unit produces a clock output signal on the basis of the counting of the timer unit. According to this method, for generating an output wave whose duty ratio is, for example, about 50%, the timer unit is set to a period that is 1/2 of a cycle of a desired output wave. The logic level of an output is reversed according to a signal representing the result of the counting performed by the timer unit.
Whatever value is set in the timer unit, the cycle of a clock output signal produced is an even multiple of the pulse spacing of the reference clock (that is, an even number of pulse spacings of the reference clock).
As mentioned above, according to the known art for generating a clock signal, the cycle or frequency of a clock output signal produced is limited to an even multiple of pulse spacings of an input clock, that is, a reference clock. It is highly probable that the frequency of an actually-obtained clock output signal is different from the desired output frequency.
In particular, when it comes to higher frequencies, the frequency of an actually-obtainable clock output signal differs greatly from the desired output frequency. This poses a problem in that an ongoing demand for a high-precision clock signal to be employed in a communication system or the like cannot be met.